Network Working Group C. Daboo
Internet-Draft Apple
Updates: 5545,6350 (if approved) October 22, 2012
Intended status: Standards Track
Expires: April 25, 2013
Parameter Value Encoding in iCalendar and vCarddraft-daboo-ical-vcard-parameter-encoding-02
Abstract
This specification updates the iCalendar and vCard data formats to
allow parameter values to include certain characters forbidden by the
existing specifications.
Status of this Memo
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Internet-Draft Parameter Encoding October 20121. Introduction
The iCalendar [RFC5545] specification defines a standard way to
describe calendar data. The vCard [RFC6350] specification defines a
standard way to describe contact data. Both of these use a similar
text-based data format. Each iCalendar and vCard data object can
include "properties" which have "parameters" and a "value". The
value of a "parameter" is typically a token or URI value, but a
"generic" text value is also allowed. However, the syntax rules for
both iCalendar and vCard prevent the use of a double-quote character
or control characters in such values, though double-quote and some
subset of control characters are allowed in the actual property
values.
As more and more extensions are being developed for these data
formats, there is a need to allow at least double-quotes and line
feeds to be included in parameter values. The \-escaping mechanism
used for property text values is not defined for use with parameter
values and cannot be easily used in a backwards compatible manner.
This specification defines a new character escaping mechanism,
compatible with existing parsers and chosen to minimise any impact on
existing data.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. Parameter Value Encoding Scheme
This specification defines the ^ character (U+005E - Circumflex
Accent) as an escape character in parameter values whose value type
is defined using the "param-value" syntax element (Section 3.1 of
iCalendar [RFC5545] and Section 3.3 of vCard [RFC6350]). The
^-escaping mechanism can be used when the value is either unquoted or
quoted (i.e., whether or not the value is surrounded by double-
quotes). When used, the combination of ^ plus one other character is
interpreted as a single character as per the table below:
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Internet-Draft Parameter Encoding October 2012
+---------------------+----------------------------+
| Escape Sequence | Character |
+---------------------+----------------------------+
| ^' (U+005E, U+0027) | U+0022 (Quotation Mark) |
| ^n (U+005E, U+006E) | U+000A (Line Feed) |
| ^^ (U+005E, U+005E) | U+005E (Circumflex Accent) |
+---------------------+----------------------------+
When generating iCalendar or vCard parameter values, the characters
in the second column above MUST be escaped using the sequence from
the first column. When parsing iCalendar or vCard parameter values,
the two character sequence from the first column MUST be replaced by
the single character from the second column. If a ^ (U+005E)
character is followed by any other character than the ones above,
parsers SHOULD leave the ^ character in place.
In addition to the escaping procedure outlined above, text parameter
values MUST also have line break character sequences canonicalized
into a single line feed (U+000A) character when the parameter value
is generated. Specifically, any single carriage return character
(U+000D) or the two character combination of carriage return followed
by line feed (U+000D, U+000A) MUST be mapped to ^n in the generated
parameter value. When parsing a parameter value with ^n, clients can
replace that with the appropriate line break character sequence for
the system they are running on.
3.1. iCalendar Example
An "ATTENDEE" property with a "CN" parameter whose value includes two
double-quote characters. The parameter value is not quoted as there
are no characters in the value that would trigger quoting as required
by iCalendar.
ATTENDEE;CN=George Herman ^'Babe^' Ruth:mailto:babe@example.com
The unescaped parameter value is
George Herman "Babe" Ruth
3.2. vCard Examples
A "GEO" property with a "X-ADDRESS" parameter whose value includes
several line feed characters. The parameter value is also quoted
since it contains a comma which triggers quoting as required by
vCard.
GEO;X-ADDRESS="Pittsburgh Pirates^n115 Federal St^nPitt
sburgh, PA 15212":geo:40.446816,-80.00566
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Internet-Draft Parameter Encoding October 2012
The unescaped parameter value is (where each line is terminated by a
line break character sequence)
Pittsburgh Pirates
115 Federal St
Pittsburgh, PA 15212
4. Security Considerations
There are no additional security issues beyond those of iCalendar
[RFC5545] and vCard [RFC6350].
5. IANA Considerations
There are none.
6. Acknowledgments
Thanks to Michael Angstadt, Mike Douglass, Barry Leiba, Simon
Perreault for feedback on this specification.
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5545] Desruisseaux, B., "Internet Calendaring and Scheduling
Core Object Specification (iCalendar)", RFC 5545,
September 2009.
[RFC6350] Perreault, S., "vCard Format Specification", RFC 6350,
August 2011.
Appendix A. Choice of Quoting Mechanism
Having recognized the need for escaping parameters values, the
question is what mechanism to use? One obvious choice would be to
adopt the \-escaping used for property values. However, that could
not be used as-is, because it escapes a double-quote as the sequence
of \ followed by double-quote. Consider what the example in
Section 3.1 might look like using \-escaping:
ATTENDEE;CN="George Herman \"Babe\" Ruth":mailto:babe@example.com
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Internet-Draft Parameter Encoding October 2012
Existing iCalendar/vCard parsers know nothing about escape sequences
in parameters. So they would parse the parameter value as:
George Herman \
i.e. the text between the first and second occurrence of a double-
quote. However, the text after the second double-quote ought to be
either a : or a ; (to delimit the parameter value from the following
parameter or property) but is not - so the parser could legitimately
throw an error at that point because the data is syntactically
invalid. Thus for backwards compatibility reasons, a double-quote
cannot be escaped using a sequence that itself includes a double-
quote. Hence the choice of using a single-quote in this
specification.
Another option would be to use a form of \-escaping modified for use
in parameter values only. However, some incorrect, non-interoperable
use of \ in parameter values has been observed, and thus it is best
to steer clear of that to achieve guaranteed, reliable
interoperability. Also, given that double-quote gets changed to
single-quote in the escape sequence for a parameter, but not for a
value, it is better to not give the impression that the same escape
mechanism (and thus code) can be used for both (which could lead to
other issues such as an implementations incorrectly escaping a ; as
\; as opposed to quoting the parameter value).
The choice of ^ as the escape character was made based on the
requirement that an ASCII symbol (non-alphanumeric) character be
used, and it ought to be one least likely to be found in existing
data.
Appendix B. Change History (To be removed by RFC Editor before
publication)
Changes in -02:
1. Removed unnecessary HTAB escaping.
2. Added appendix to describe why ^ was chosen as the escape
mechanism.
Changes in -01:
1. Tweaked vCard example line-folding.
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